1. Field of the Invention
Embodiments of the present invention generally relate to wireless communications and more particularly to dual mode power amplifiers.
2. Description of the Related Art
Wireless communications systems generally use radio frequency (RF) signals to transmit data from a transmitter to one or more receivers. Wireless communication systems are frequently used to implement wireless local area networks (LANs) in which data is transmitted and received between computers, servers, Ethernet switches, hubs, and the like.
There are many modulation schemes that may be used to encode and transmit data. Bluetooth™ devices, for example, may use three different modulation schemes as currently set forth in the Bluetooth SIG specification v2.0.
A first modulation scheme uses Gaussian frequency-shift keying (GFSK) which encodes data by modulating a carrier frequency of between 2402 and 2480 MHz by +/−160 KHz. This modulation scheme is one of the relatively early schemes employed for earlier Bluetooth™ products. This modulation scheme may transmit up to 1 Mbs.
A second modulation scheme, 4-Differential Phase Shift Keying (DPSK) encodes data by modulating the phase of a carrier frequency to one of four possible phase settings. The 4-DPSK modulation scheme can support up to 2 Mbs of throughput. A third modulation scheme, 8-DPSK encodes bits by modulating the phase of a carrier frequency to one of eight fixed phases and can support up to 3 Mbs of throughput. Again, in the second and third modulation schemes, the carrier frequency may be between 2402 and 2480 MHz.
The GFSK modulation scheme may be realized in hardware with a relatively simple non-linear power amplifier. One example of a non-linear power amplifier is a switching power amplifier. Switching power amplifiers advantageously use relatively small amounts of power. FIG. 1 is a conceptual drawing of a switching power amplifier 100. To transmit a GFSK modulated signal, a desired modulating frequency f(t) is coupled to the input of switching power amplifier 100, as shown. In some embodiments, switching power amplifier 100 may apply an amount of gain to the coupled input. In this exemplary switching power amplifier, an amount of gain A is applied to the modulating frequency f(t), thereby producing an output of Af(t). The output of switching power amplifier 100 may be coupled to an antenna 101. Switching power amplifier 100 works relatively well for transmitting GFSK modulated signals because the amplitude of the transmitted signal is relatively constant while the frequency of the transmitted signal changes in response to the modulating frequency f(t).
In contrast, the 4-DPSK and 8-DPSK modulation schemes are often realized with linear power amplifiers. The 4-DPSK and 8-DPSK modulation schemes are relatively more complicated than GFSK modulation and generally use linear power amplifiers configured with either Cartesian or polar inputs. A Cartesian amplifier typically includes i(t) and q(t) modulating inputs while a polar amplifier typically includes frequency f(t) and amplitude a(t) inputs. FIG. 2 is a conceptual diagram of a linear polar amplifier 200 with frequency f(t) and amplitude a(t) inputs. As is well-known, the output of a linear polar amplifier may be described by the equation a(t)×f(t). One drawback of a linear amplifier is that it typically uses relatively more power compared to a non-linear amplifier (e.g. switching power amplifier 100). Using more power may be undesirable, especially for battery-powered transmitter designs.
In some designs when it may be desirable to support both GFSK and DPSK modulation schemes, it may be useful to include both a non-linear power amplifier along with a linear power amplifier in the design. The non-linear power amplifier may provide a power-saving amplifier for GFSK signals, while the linear power amplifier may be used for DPSK modulation schemes. A drawback to this approach is that additional die area may be required to support both power amplifier designs in a single integrated circuit.
As the foregoing illustrates, what is needed in the art is a power amplifier that supports both relatively low power GFSK modulation as well as 4-DPSK and 8-DPSK modulation while using relatively little die area.